BRUSSELS, MARCH 3 -- Margaret Thatcher fixed one of her most withering glares on the British journalist who was asking if the formula adopted at the NATO summit for future arms control negotiations with the Soviet Union was not far more ambiguous than the British prime minister had desired.

"I am clear" about the policy, Thatcher said with ice in her voice. "The United States is clear." Her tone left her audience in no doubt that the other alliance leaders could, in her view, like it or lump it.

That moment in a Thatcher news conference captured some of the major themes of the two-day summit of 16 national leaders of the North Atlantic Treaty Organization that ended here today.

It was a summit that was transformed by the force and doggedness of Thatcher, who dominated proceedings that normally revolve around an American leader's program. The issue she and the journalists were discussing involved battlefield nuclear weapons, which gained new importance here.

Most importantly, it was a meeting that papered over rather than resolved serious differences and left each participant free to insist, as Thatcher was doing, that his or her interpretation of the conference's ambiguities was the only valid one.

For the summit's most important nonparticipant, whose actions and words guided these deliberations to a great extent, the mixed messages of Brussels undoubtedly will be more welcome than not.

That nonparticipant was Soviet leader Mikhail Gorbachev, who obtained some mild praise and direct responses on substance from this first NATO summit since he came to power in 1985. Except for a renewed NATO commitment to nuclear weapons, which Gorbachev continues to say must be abolished, much of the Kremlin leader's arms control agenda was confirmed by the decisions here.

A formal declaration by the 16 leaders welcomed "encouraging signs of change in the policies of the Soviet Union and some of its allies," which create "the prospect for greater openness in their relations with their own peoples and with other nations."

Thatcher went even further in her postsummit news conference remarks, praising Gorbachev's "bold and courageous reforms," which she said were producing freer conditions inside the Soviet Union.

She also reminded her audience that she was the first western head of government "to say that he was a different kind of Soviet leader" and to extend public support for his domestic reform program.

She strongly denied that there was any inconsistency between her open admiration for Gorbachev as a "confident and resolute" leader and the tough language on defense that she insisted on inserting in the two statements that were formally adopted and issued by the summit.

The NATO leaders' welcome of "signs of change" was immediately followed, for example, by a pledge to remain vigilant because "we have to date witnessed no relaxation of the military effort pursued for years by the Soviet Union." The "massive" Soviet military force "constitutes a fundamental source of tension between East and West," the document added.

Thatcher said that NATO could afford to pursue a policy of "defense, deterrence and dialogue" with Gorbachev's Soviet Union from a position of strength, which had to be based on a credible nuclear arsenal. "I can do it, NATO can do it, if our insurance policy is sure," she explained.

Thatcher's pursuit of alliance guarantees that battlefield nuclear weapons would be modernized and fenced off from new negotiations for now overshadowed U.S. hopes for a celebration of President Reagan's Intermediate-range Nuclear Forces (INF) treaty with Gorbachev and Reagan's upcoming trip to Moscow.

Those questions received relatively little attention, and the American role here seemed to have been passive both in the closed meetings and public sessions.

In contrast to Thatcher's domineering performance and the deft presentations made by West German Chancellor Helmut Kohl and French President Francois Mitterrand at the close of the summit, Reagan took no questions from the press and his appointed spokesman, Secretary of State George P. Shultz, responded to only three questioners.

Kohl was able to leave the conference arguing that it had been a holding operation and that West Germany had avoided giving the new commitments Thatcher had sought on accepting deployment of a modern version of the U.S. short-range Lance missile.

Thatcher, who is prepared to increase the number of nuclear-capable U.S. bombers stationed in Britain to compensate for the striking force lost under the INF treaty, apparently wants other countries to agree now to modernize U.S. forces on their territory, as well, in a demonstration of risk-sharing, diplomatic analysts believe.

The summit declaration also suggests that the Europeans feel they have reined in Reagan's perceived nuclear utopianism. In his October 1986 summit with Gorbachev in Iceland, Reagan repeatedly castigated nuclear weapons as immoral and said they must be abolished.

But Reagan subscribed to a statement on conventional arms control issued yesterday that committed the alliance to keeping nuclear weapons in Europe even if the Soviets' large numerical superiority in conventional forces were to be eliminated through arms control negotiations.

"Although conventional parity would bring important benefits for stability, only the nuclear element can confront a potential aggression with an unacceptable risk; therefore, for the foreseeable future deterrence will continue to require an adequate mix of nuclear as well as conventional forces," the statement said.

This formulation offers a new wrinkle in the alliance's "flexible response" doctrine, introduced in the mid-1960s in part to ensure that the United States would not have to respond automatically to a conventional Soviet attack by immediately using nuclear weapons.

In other ways the conventional arms statement represented movement by NATO toward the kind of serious negotiations on reducing Warsaw Pact and NATO nonnuclear forces that Gorbachev has said repeatedly he is ready to conduct.

The alliance used the statement to shift the emphasis of conventional arms control from the counting and demobilizing of troops to removing weapons systems that would be used in any surprise attack or large-scale offensive actions, such as tanks and artillery.

The statement does not rule out the inclusion in such negotiations of artillery or aircraft that can use either conventional or nuclear weapons. Warsaw Pact officials have identified these "dual capable" systems as weapons they would particularly like to see abolished.